<国立科博専報>小笠原固有種子植物の細胞分類学的研究
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概要
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Concerning the flowering plants of the Bonin Islands, a high degree of endemism has been noticed by many taxonomists, i.e., nearly 53% in total vascular plants (YAMAZAKI 1970), or more than 67% in arboreal species (TUYAMA 1968). In order to redefine the circumscription of these endemic 'species', I have studied the comparison of chromosome numbers and karyotypes of these entities whth those of their adjacent or taxonomically close related counterparts. Herewith I report the chromosome numbers of some 50 species(including one variety) of flowering plants (mostly trees) endemic to the Islands. The materials examined are listed in Table 1. The results are summarized in 4 groups as shown in Table 2,namely, A) the Bonin species possessing the same chromosome numbers as their adjacent or correspondent species within the genus (22 species), B) the Bonin species being considered as a polyploid of their adjacent ones, (polyploid group, 7 spp.), C) the Bonin species possessing a different chromosome numbers from their counterparts (aneuploid group, 14 spp.) and D) the species belonging to the genus of which the chromosome number has not been counted yet (7 species and 1 variety). In conclusion, the species belonging to the C) group are worthy to note : in these species (or variety), the somatic chromosome numbers show slight differences, beeing mostly one pair or two pairs less (or more) than those of the adjacent or other congeneric species. This fact seems to me to suggest an important role of aneuploidy in the process of speciation of these plants in the Bonin Islands. In addition, it is noteworthy that, the all three species of Crepidiastrum (Compositae, Ixeris sensu lato.) of the Islands have 2n=10 chromosomes, as same number as their counterparts occuring in Japan Proper. It supports the taxonomy of T. NAKAI (1920) who recognized a seperate genus Crepidiastrum from Ixeris in which chromosome number is counted as n=7 or 8. As for the D) group, except two genera being endemic to the Islands (Boninia and Dendrocacalia), each species has several congeneric counterparts in South East Asia including the Ryukyu Islands. Thus by further investigation on those materials, the D) grouped species would become to be included in one of the other three groups.
- 1977-12-20